Is it safe to flash Stock ROM on an unlocked Tab S2?

I have just got a pre-owned Tab S2 T817W which is a Canadian version from Bell Canada.

It has been unlocked and I can put in another carrier's SIM card and use it.

However, it is in stock 5.1.1 stock ROM and I want to upgrade to the latest Marshmallow.

My questions are:
1) If I flash the latest ROM from Bell for this T817W, will the ROM lock it back to Bell only?

2) Can I flash other carrier's ROM, eg T817A (for ATT) T817V (for Verizon) etc?

3) Do I need to root it before upgrading to Marshmallow?

Thanks.

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